An immunoassay analyzer is highly automatic, where its liquid surface detection device can automatically detect a sample so that the sample can be added into a test position. The liquid surface detection device can mainly include three parts: a signal analysis circuit, a sample probe and a fluidic system, where the fluidic system can connect with the sample probe. When the sample probe reaches a liquid surface of the sample, the signal analysis circuit can recognize a liquid surface signal, so that the sample can be drawn and discharged thereafter. Each time the sample probe completes an absorption-discharging operation, it cleans the sample probe to prevent contaminating the next sample. A method for cleaning the sample probe commonly used today may immerse the sample probe in a cleaning fluid for cleaning.
In a fully automatic immunoassay analyzer, the liquid surface may generally be detected through a capacitance liquid surface detection technology. At this point, the sample probe may be connected in the signal analysis circuit as a variable capacitor, in which case the sample probe can have different capacitances with respect to different liquid surfaces and the signal analysis circuit can monitor changes in the capacitance of the sample probe. When the sample probe reaches the liquid surface, its capacitance may have changed and the signal analysis circuit can accurately recognize the liquid surface signal and determine whether the sample probe reaches the liquid surface by detecting a phase change caused by the changes in the capacitance of the sample probe.
The existing liquid surface detection methods are simple and reliable and have high detection sensitivity. However, during the use of the automatic immunoassay analyzer, since the cleaning fluid for cleaning the sample probe has a high ion concentration, electrochemical reactions can occur between the cleaning fluid and the sample probe, and the sample probe may get corroded by the cleaning fluid and become rusted after several cleanings, thereby requiring frequent replacement of the sample probe.
Such electrochemical corrosion may be caused by the following reasons: when impure metal (or alloy) is immersed within an electrolyte solution, active metal may lose electrons and be oxidized, so that primary battery reactions may occur to cause the corrosion. According to a primary battery model, two conditions are required to cause the electrochemical corrosion: (1) a medium (such as the cleaning fluid above) having a high ion concentration and (2) metal material with low purity.
Therefore, based on the above-described analysis, the rusting problem of the sample probe can be solved in view of two aspects as follows. In a first aspect, the cleaning fluid can be replaced by another liquid with a low ion concentration. Deionized water, for example, can be used to clean the sample probe to remove electrically conductive material. In a second aspect, the sample probe with high metal purity can be adopted. Up to now, when the liquid surface is detected using the capacitance liquid surface detection technology, the rusting problem of the sample probe are addressed by the two aspects described above; that is, the cleaning fluid or the metal material for the sample probe is replaced. These two aspects have their own drawbacks, however. For instance, when the deionized water, which lacks strong detergency, is used for cleaning, the sample probe may not be cleaned enough, and some residual contamination may exist to affect the overall performance of the analyzer. On the other hand, when the sample probe is instead made by some special metal with high purity, such as Ti, the rusting problem can be overcome, but new problems about manufacture cost may arise because that metal is very expensive and hard to process.
Besides, the liquid surface can be detected by technologies other than the capacitance liquid surface detection technology. As an example, a radio frequency detection method can be used to detect the liquid surface. However, this technology is complicated in structure and easy to be interfered.